A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane 102 which moves the bones of the middle ear 103 that vibrate the oval window opening of the cochlea 104. The cochlea 104 is a long narrow duct that is wound spirally about a central axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ear's ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, hearing prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system. An external microphone provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format such as a sequence of data frames for transmission into an implanted stimulator 108. Besides receiving the processed audio information, the implanted stimulator 108 may also perform additional signal processing such as error correction, pulse formation, etc. and produces stimulation signals (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 112. The electrode array 112 includes multiple electrode contacts 110 on its surface that deliver the stimulation signals to adjacent neural tissue of the cochlea 104 which the brain interprets as sound. The individual electrode contacts 110 may be activated sequentially, or simultaneously in one or more groups of electrode contacts 110.
Though existing commercial products generally use an external microphone to sense the acoustic environment, there would be an advantage to an implantable microphone. Among other things, an implantable microphone needs to be hermetically sealed from the environment of the adjacent tissue. This makes implantable microphones very sensitive to pressure changes such as those that occur on an airplane flight. For high acoustic sensitivity, a good acoustic impedance match to the surrounding tissue is necessary, which due to the required hermeticity is achieved using thin metal membranes. Pressure changes on the outside of the microphone relative to the hermetic inside of the microphone then can cause a deformation of the membrane, which produces bad microphone characteristics or even a total microphone failure. This problem can be mitigated somewhat by increasing the pressure inside the hermetic microphone housing and thus shifting the pressure range. But the change with pressure in the microphone characteristics remains unsolved.